The invention relates to an intervertebral disc prosthesis, intended to be substituted for fibrocartilaginous discs providing the liaison between the vertebrae of the spinal column.
Different types of intervertebral disc prostheses are known in the prior art. Numerous prostheses, such as for example those described in the applications WO 02 089 701 and WO 2004/041129, are constituted of a lower plate and an upper plate creating a cage articulated about a central core. Other prostheses like those disclosed in the patent U.S. Pat. No. 5,676,701 and in the application WO 03/059212 A1, for example, only comprise a lower plate and an upper plate articulated about themselves by means of a surface of articulation. These articulated prostheses have the advantage of offering the patient bearing the prosthesis a freedom of movement, by allowing the plates to tilt and/or rotate in relation to each other. The prostheses comprising a central core, movable between the plates, have the added advantage of allowing a spontaneous positioning of the core in the ideal position for absorbing the constraints imposed on the prosthesis. In these prostheses known in the prior art, the anterior, posterior and lateral edges of a plate are located on the same vertical axis as the corresponding edge of the other plate. This shape of the prosthesis is normally due to the plates being of identical size and that their respective axes of articulation are joined (coaxially), so as to facilitate the movements of the patient and to allow the correction of possible positioning defects. However, these prostheses have the inconvenience of not being perfectly suited to the morphology of the spinal column. Indeed, the posterior edges of two adjacent vertebrae are often slightly off-set to each other. Thus, the prostheses known in the prior art are difficult to properly implant. Additionally, at rest, due to the natural off-setting of the vertebrae and the anchoring of the plates in the vertebrae, the different parts of the prosthesis are under constraint in an undesirable position as it restricts freedom of movement of these parts of the prosthesis. This inconvenience will be diminished through the use of a movable core between the plates, but the possible movements of the core will be restricted and its capacity to position itself so as to absorb the constraints imposed on the prosthesis will therefore be diminished.
In this context, it is beneficial to propose a prosthesis that allows a more efficiently fit to the profile of the spinal column and thus fully attain the goals it set by offering a surface of articulation.